1. Field of the Invention
This invention relates generally to the turbine section of a gas turbine, and more particularly to an apparatus and method for supporting the torque load on the turbine vanes.
Gas turbines employ a row of stationary vanes immediately upstream of each row of rotating blades to properly direct the hot gas flow to the rotating blades. A row of vanes comprises a plurality of vanes arrayed circumferentially around the flow path annulus. The vanes are retained in a turbine cylinder. To allow access to the turbine components, the turbine cylinder is split longitudinally into semi-circular upper and lower halves. The halves are joined together at horizontal joints by a plurality of bolts disposed along flanged portions of the cylinder halves extending the length of each cylinder.
Each vane comprises an airfoil, an outer shroud at its radially outboard end and an inner shroud at its radially inboard end. The flow of hot gas over the airfoil generates an axial force tending to urge the vane downstream. A support rail emanating radially from each outer shroud serves to restrain the motion of the vane in the axial direction by mating with the inner edge of a plate which is affixed at its outer edge to the turbine cylinder. Additional axial restraint may be obtained by a second support rail in the inner or outer shroud.
The flow of hot gas over the airfoil also generates a torque load on the vane, tending to urge it circumferentially around the turbine annulus. This load is absorbed by torque pins which engage the outer shroud support rail and transmit the load to the turbine cylinder. However, changes in the design of the turbine cylinder necessitated by the high temperature of modern gas turbines have made it impractical to use torque pins of the traditional type in the flanged area of the horizontal joints. This invention relates to a new type and method of using a torque pin which is suitable for use in the flanged areas.
2. Description of the Prior Art
In the past, torque pins were inserted and removed from outside of the cylinder through holes in the cylinder. This allowed the vanes to be removed without opening the turbine to disengage the pins. The pin was of cylindrical shape, a key being formed on one end adapted to engage a key-way in the vane outer shroud support rail. At installation the pin was inserted into the hole in the cylinder, pushing it radially inward until it engaged the vane. Since the diameter of the pin was only slightly smaller than that of the hole, the torque load on the vane was transmitted through the outer shroud and pin to the cylinder. A head formed on the end of the pin protruded through the hole in the cylinder and was seated in a counterbore on the outer surface of the cylinder. Motion in the radially outboard direction was prevented by a retainer bracket, affixed to the cylinder with screws, which spanned the head of the pin protruding from the cylinder, thus preventing the pin from accidentally disengaging.
In addition to the vanes, the turbine cylinder also contains a plurality of segments, arrayed around the turbine annulus, which form a ring encasing the tips of the rotating blades. To obtain optimum thermodynamic performance, the radial clearance between these segments and the tips of the rotating blades is maintained at a minimum. Hence, it is important that the turbine cylinder retain as nearly a perfectly cylindrical shape as possible. As a result of increases in the temperature of the hot gas flowing in the turbine cylinders of modern gas turbines, thermal stresses in the cylinder can cause it to ovalize and adversely affect the radial clearance between the segments and the rotating blades. To prevent this occurrence the thickness of the cylinder flanges and the diameter of the joint bolting has been increased and the spacing of the joint bolting has been decreased. As a result of these changes, however, there is insufficient clearance between the joint bolting to insert torque pins from outside of the cylinder to engage the vanes in the vicinity of the joint.
It is therefore desirable to provide an apparatus and method for transmitting the torque load on the vanes in the vicinity of the cylinder joint through pins which, although inserted in the turbine cylinder, do not interfere with the joint bolting and can be disengaged from outside the cylinder.